Tesla battery secrets revealed: How to increase power

        Recently, Tesla's 100kWh model has passed the evaluation of the EU certification agency RDW. This means that the Model S/X 100D model is about to be released! Its theoretical range will reach 613km (based on the NEDC standard).

　　
       According to EU regulations, all models sold in EU member states must be certified by their authorized agencies. RDW is a Dutch company commissioned by Tesla. After certification, it can obtain permission to sell in the EU. Let's explore how this 100kWh is achieved?

　　Elon Musk once said that Tesla's battery life (power) should increase by 5% every year. Judging from the current iteration of the battery pack, this goal has been basically achieved. In addition to the 60kWh as the entry-level configuration, the 70kWh and 85kWh have been upgraded to 75kWh and 90kWh respectively.

　　
       Soon, 100kWh and 120kWh battery packs will also be included in the optional list. Currently, the 60kWh is still a beggar version configuration to promote Tesla's sales. The real story is how the 70kWh and 85kWh each increase the power by 5kWh.

　　
       One thing is certain, that is, the structure of the battery pack has not changed during the process of increasing the battery capacity. The number of internal battery modules has not changed. Let's first take a brief look at the internal structure of Tesla's battery pack.

　　
       The 60kWh has 14 battery packs, each containing 384 battery cells, for a total of 5376 battery cells; the 85kWh consists of 16 battery packs, each containing 444 battery cells, for a total of 7102 battery cells.

　　
       The 70kWh added later is actually a 75kWh battery pack that has been restricted by software. The extra 5kWh was originally provided to car owners as an optional package worth $3,000. As long as the OTA software update is performed, the 70D can be turned into a 75D.

　　
       So the question is, how did the 75kWh battery pack come from? Tesla officials have not given a technical explanation for this question. According to the author's judgment, the 75kWh is actually an 85kWh battery pack, minus 2 battery packs. In the 85kWh battery, the capacity of each battery pack is 5.3kWh, and 14 such battery packs are 74.2kWh.

　　
       This is the relationship between 70kWh, 75kWh, and 85kWh. As for 60kWh, it is just a configuration set to lower the entry threshold. So, how did 90kWh come about?

　　
       From 85kWh to 90kWh, there is an increase of 5kWh. Is it because of the addition of an extra battery pack? In the 85kWh battery pack structure, it is no longer possible to stack battery packs. The only possibility is that new battery cells have been replaced. Of course, it still uses the 18650 battery cell, but the chemical material has been adjusted to increase the energy density.

　　
       During this process, Tesla added a small amount of silicon to the graphite anode of the battery cell, thereby increasing the energy density of the battery cell.

　　
       Adding silicon to the anode is a well-known method in the battery field to increase energy density. In order to avoid excessive weight of the battery pack caused by continuous stacking of battery packs, Tesla can only focus on the research and development of high energy density cells. However, for ternary lithium-ion batteries, it is far from easy to increase energy density through silicon.

　　
       The basic principle is that after adding silicon to the graphite anode, the structure of silicon atoms can accommodate more lithium ions than graphite, resulting in an enhanced anode's ability to absorb lithium ions. In a single charge and discharge cycle, the more lithium ions in the anode, the greater the energy density.

　　
       However, after silicon fully absorbs lithium ions, its volume will expand by 300%, which is much larger than the 7% expansion rate of graphite after absorbing lithium ions. This repeated volume change will cause the solid electrode to become "soft" and easy to collapse. As a result, the cycle life of the battery will be reduced.

　　
       Another factor is that the expansion/contraction characteristics of the silicon anode during charging and discharging will destroy the formation of the SEI film of the lithium battery electrolyte. This film is formed during the initial cycle of the lithium battery and has a protective effect on the anode material, preventing the material structure from collapsing.

　　
       Based on the above reasons, although the energy density can be significantly improved by using silicon materials as anodes, it is also accompanied by side effects, which will eventually shorten the battery life. Therefore, Tesla's solution is to gradually add a small amount of silicon to the graphite anode to find a balance between energy density and cycle life.

　　
       As we all know, the 18650 batteries used by Tesla are produced by Panasonic. As the cooperation between the two parties deepens, Tesla is also developing new cylindrical batteries. After the Model 3 is officially put into production, the new 21700 battery will replace the 18650 and become the new battery cell.

　　
       The 21700 battery is still a ternary lithium battery, and the cathode material is nickel cobalt aluminum oxide (NCA). This cylindrical ternary battery is currently the power battery solution with the highest energy density. Compared with square batteries, this type of battery has a high energy density but poor stability and requires a better BMS (battery management system) support.

　　
        Tesla's earliest Roadster used Panasonic's NCR18650A battery, rated at 3.6V and 3.1Ah. The previous 85kWh battery pack used NCR18650B battery, rated at 3.6V and 3.1Ah.

　　
       The model of the 90kWh battery is unknown, but it should not be a finished product directly provided by Panasonic, but a customized battery cell jointly developed by Tesla and Panasonic for Tesla models. Currently, among the 18650 batteries produced by Panasonic, the NCR18650G type has the highest capacity, reaching 3.6Ah. If calculated according to this, the 7102 cells in the 85kWh battery pack are replaced with G-type batteries, which is exactly 90kWh.

　　
       Therefore, one possibility is that the cells in the 90kWh battery pack are NCR18650G, while the cells in the 85kWh battery pack are NCR18650B. In short, if the number of cells remains unchanged (battery pack structure remains unchanged), only by increasing the capacity of a single cell to 3.6Ah can the 90kWh power be guaranteed.

　　
       To achieve 100kWh, there are two options: one is to stack two more battery packs, and according to the capacity of each battery pack of 5.3kWh, you can get exactly 100kWh; the other is to replace the battery cell with a higher energy density. The author believes that the latter is the best and most likely option.

　　
       Because the 90kWh is based on the 85kWh battery pack structure. This structure has been finalized under the 18650 battery specification, and the cost of changing its design structure is very high. In fact, there is no room in the battery pack to stack more battery packs.

　　
       If more battery packs are added, not only will the mass of the battery pack increase, but the cooling circulation system of the battery pack will also need to be changed. Therefore, increasing the capacity of the battery cell is the most economical and feasible solution.

　　
       Imagine that in a 100kWh battery pack, without changing the battery pack structure, the capacity of a single cell must be increased to 3.9Ah to achieve a capacity of 100kWh. Therefore, the author speculates that Tesla has developed a 3.9Ah 18650 cell with Panasonic. This credit can only be attributed to the silicon in the anode.